If OMAPI is working properly you can test failover by stopping the primary server.

If you are using a firewall, you will need to open TCP ports 647 and 7911

Once you are sure everything is working as expected, restart both servers to ensure everything is running correctly.

The 'split' value '128' on the 'primary', divides responsibility for the clients between the two failover partners.
If you want the primary to answer all dhcp requests unless it is down (for whatever reason) set the value to '255', use '0' to make the secondary responsible.